1. Field of the Invention
The present invention relates to an A/D conversion method and apparatus for converting an analog input signal to digital data using a pulse delay circuit comprised of a plurality of series-connected delay units outputting a pulse signal with a delay.
2. Description of the Related Art
Known in the past has been, as an A/D converter of a simple configuration giving high resolution digital values, an A/D converter supplying an analog input signal for A/D conversion as power source voltage to a pulse delay circuit comprised of a plurality of delay units comprised of various gate circuits connected in a ring, inputting a pulse signal for propagation to cause the pulse signal to circulate in the pulse delay circuit by a speed corresponding to the delay time of the delay units, and counting the number of delay units through which the pulse signal passes in the pulse delay circuit within a predetermined sampling time during circulation of the pulse, and thereby converting the analog input signal to digital data (for example, see Japanese Unexamined Patent Publication (Kokai) No. 5-259907).
This A/D converter utilizes the fact that the delay time of the delay units changes in accordance with the power source voltage. It supplies the analog input signal as a power source voltage to the delay units forming the pulse delay circuit to thereby modulate the speed of movement of the pulse signal circulating in the pulse delay circuit by the analog input signal, measures the speed of movement by counting the number of delay units through which the pulse signal passes in a predetermined sampling time, and outputs the results of measurement (count) as digital data after the A/D conversion.
Further, according to this A/D converter, it is possible to set the voltage resolution of the digital data obtained in accordance with the delay time per delay unit forming the pulse delay circuit and the sampling time at the time of A/D conversion and possible to raise the voltage resolution of the digital data by either shortening the delay time per delay unit or lengthening the sampling time, so it is possible to provide an A/D converter able to realize high precision A/D conversion by a simple configuration and inexpensive cost.
Summarizing the problems to be solved by the invention, in the above A/D converter, the delay time per delay unit forming the pulse delay circuit is determined by the level of miniaturization (CMOS design rule) of the device forming the delay unit (inverter or other gate circuit), so there are limits to shortening the delay time per delay unit of the pulse delay circuit for raising the resolution of the A/D conversion.
Further, in the above A/D converter, if increasing the sampling time at the time of A/D conversion for raising the resolution of the A/D conversion, in a system requiring high speed A/D conversion such as an A/D conversion speed of several MHz to several tens of MHz, the speed ends up becoming insufficient and the requirements can no longer be met.
That is, the A/D converter is a so-called integration type A/D converter. The obtained digital value is obtained by integrating the fluctuating component of the analog input signal by the sampling time of the A/D conversion, so if the sampling time of the A/D conversion is increased to raise the resolution of the A/D conversion, it is not possible to reflect any fluctuation in the analog input signal in the obtained digital value and utilization in a system requiring high speed A/D conversion ends up becoming impossible.
Therefore, conventionally, in an apparatus requiring speed and precision of A/D conversion, a successive comparison type A/D converter of a higher speed than the above integration type A/D converter or a parallel type (also called “flash” type) A/D converter enabling instantaneous A/D conversion has been used.
However, to raise the resolution of the A/D conversion in such a successive comparison type or parallel type A/D converter, it is necessary to generate a large number of reference voltages corresponding to the resolution, so there was the problem that in an apparatus requiring speed and precision of A/D conversion, it was only possible to use a complicatedly configured, expensive A/D converter and higher costs of the apparatus as a whole were invited.